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          编程风格
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      </script></div> <div class="theme-vdoing-content content__default"><h1 id="编程风格"><a href="#编程风格" class="header-anchor">#</a> 编程风格</h1> <p>本章探讨如何将 ES6 的新语法，运用到编码实践之中，与传统的 JavaScript 语法结合在一起，写出合理的、易于阅读和维护的代码。</p> <p>多家公司和组织已经公开了它们的风格规范，下面的内容主要参考了 <a href="https://github.com/airbnb/javascript" target="_blank" rel="noopener noreferrer">Airbnb<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> 公司的 JavaScript 风格规范。
</p> <h2 id="块级作用域"><a href="#块级作用域" class="header-anchor">#</a> 块级作用域</h2> <p><strong>（1）let 取代 var</strong></p> <p>ES6 提出了两个新的声明变量的命令：<code>let</code>和<code>const</code>。其中，<code>let</code>完全可以取代<code>var</code>，因为两者语义相同，而且<code>let</code>没有副作用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'use strict'</span><span class="token punctuation">;</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">10</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码如果用<code>var</code>替代<code>let</code>，实际上就声明了两个全局变量，这显然不是本意。变量应该只在其声明的代码块内有效，<code>var</code>命令做不到这一点。</p> <p><code>var</code>命令存在变量提升效用，<code>let</code>命令没有这个问题。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">'use strict'</span><span class="token punctuation">;</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// ReferenceError</span>
  <span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码如果使用<code>var</code>替代<code>let</code>，<code>console.log</code>那一行就不会报错，而是会输出<code>undefined</code>，因为变量声明提升到代码块的头部。这违反了变量先声明后使用的原则。</p> <p>所以，建议不再使用<code>var</code>命令，而是使用<code>let</code>命令取代。</p> <p><strong>（2）全局常量和线程安全</strong></p> <p>在<code>let</code>和<code>const</code>之间，建议优先使用<code>const</code>，尤其是在全局环境，不应该设置变量，只应设置常量。</p> <p><code>const</code>优于<code>let</code>有几个原因。一个是<code>const</code>可以提醒阅读程序的人，这个变量不应该改变；另一个是<code>const</code>比较符合函数式编程思想，运算不改变值，只是新建值，而且这样也有利于将来的分布式运算；最后一个原因是 JavaScript <strong>编译器会对<code>const</code>进行优化</strong>，所以多使用<code>const</code>，有利于提高程序的运行效率，也就是说<code>let</code>和<code>const</code>的本质区别，其实是编译器内部的处理不同。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">,</span> b <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">,</span> c <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>

<span class="token comment">// good</span>
<span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> c <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>

<span class="token comment">// best</span>
<span class="token keyword">const</span> <span class="token punctuation">[</span>a<span class="token punctuation">,</span> b<span class="token punctuation">,</span> c<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p><code>const</code>声明常量还有两个好处，一是阅读代码的人立刻会意识到不应该修改这个值，二是防止了无意间修改变量值所导致的错误。</p> <p>所有的函数都应该设置为常量。</p> <p>长远来看，JavaScript 可能会有多线程的实现（比如 Intel 公司的 River Trail 那一类的项目），这时<code>let</code>表示的变量，只应出现在单线程运行的代码中，不能是多线程共享的，这样有利于保证线程安全。</p> <h2 id="字符串"><a href="#字符串" class="header-anchor">#</a> 字符串</h2> <p>静态字符串一律使用单引号或反引号，不使用双引号。动态字符串使用反引号。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token string">&quot;foobar&quot;</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token string">'foo'</span> <span class="token operator">+</span> a <span class="token operator">+</span> <span class="token string">'bar'</span><span class="token punctuation">;</span>

<span class="token comment">// acceptable</span>
<span class="token keyword">const</span> c <span class="token operator">=</span> <span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">foobar</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">;</span>

<span class="token comment">// good</span>
<span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token string">'foobar'</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">foo</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>a<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">bar</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><h2 id="解构赋值"><a href="#解构赋值" class="header-anchor">#</a> 解构赋值</h2> <p>使用数组成员对变量赋值时，优先使用解构赋值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token comment">// bad</span>
<span class="token keyword">const</span> first <span class="token operator">=</span> arr<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> second <span class="token operator">=</span> arr<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token comment">// good</span>
<span class="token keyword">const</span> <span class="token punctuation">[</span>first<span class="token punctuation">,</span> second<span class="token punctuation">]</span> <span class="token operator">=</span> arr<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>函数的参数如果是对象的成员，优先使用解构赋值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">function</span> <span class="token function">getFullName</span><span class="token punctuation">(</span><span class="token parameter">user</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> firstName <span class="token operator">=</span> user<span class="token punctuation">.</span>firstName<span class="token punctuation">;</span>
  <span class="token keyword">const</span> lastName <span class="token operator">=</span> user<span class="token punctuation">.</span>lastName<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// good</span>
<span class="token keyword">function</span> <span class="token function">getFullName</span><span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> <span class="token punctuation">{</span> firstName<span class="token punctuation">,</span> lastName <span class="token punctuation">}</span> <span class="token operator">=</span> obj<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// best</span>
<span class="token keyword">function</span> <span class="token function">getFullName</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span> firstName<span class="token punctuation">,</span> lastName <span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>如果函数返回多个值，优先使用对象的解构赋值，而不是数组的解构赋值。这样便于以后添加返回值，以及更改返回值的顺序。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">function</span> <span class="token function">processInput</span><span class="token punctuation">(</span><span class="token parameter">input</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token punctuation">[</span>left<span class="token punctuation">,</span> right<span class="token punctuation">,</span> top<span class="token punctuation">,</span> bottom<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// good</span>
<span class="token keyword">function</span> <span class="token function">processInput</span><span class="token punctuation">(</span><span class="token parameter">input</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token punctuation">{</span> left<span class="token punctuation">,</span> right<span class="token punctuation">,</span> top<span class="token punctuation">,</span> bottom <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> <span class="token punctuation">{</span> left<span class="token punctuation">,</span> right <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token function">processInput</span><span class="token punctuation">(</span>input<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><h2 id="对象"><a href="#对象" class="header-anchor">#</a> 对象</h2> <p>单行定义的对象，最后一个成员不以逗号结尾。多行定义的对象，<strong>最后一个成员以逗号结尾</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token punctuation">{</span> k1<span class="token operator">:</span> v1<span class="token punctuation">,</span> k2<span class="token operator">:</span> v2<span class="token punctuation">,</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token punctuation">{</span>
  k1<span class="token operator">:</span> v1<span class="token punctuation">,</span>
  k2<span class="token operator">:</span> v2
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// good</span>
<span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token punctuation">{</span> k1<span class="token operator">:</span> v1<span class="token punctuation">,</span> k2<span class="token operator">:</span> v2 <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token punctuation">{</span>
  k1<span class="token operator">:</span> v1<span class="token punctuation">,</span>
  k2<span class="token operator">:</span> v2<span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>对象尽量静态化，一旦定义，就不得随意添加新的属性。如果添加属性不可避免，要使用<code>Object.assign</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
a<span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>

<span class="token comment">// if reshape unavoidable</span>
<span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span>a<span class="token punctuation">,</span> <span class="token punctuation">{</span> x<span class="token operator">:</span> <span class="token number">3</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// good</span>
<span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token punctuation">{</span> x<span class="token operator">:</span> <span class="token keyword">null</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
a<span class="token punctuation">.</span>x <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>如果对象的属性名是动态的，可以在创造对象的时候，使用属性表达式定义。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
  id<span class="token operator">:</span> <span class="token number">5</span><span class="token punctuation">,</span>
  name<span class="token operator">:</span> <span class="token string">'San Francisco'</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
obj<span class="token punctuation">[</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token string">'enabled'</span><span class="token punctuation">)</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>

<span class="token comment">// good</span>
<span class="token keyword">const</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
  id<span class="token operator">:</span> <span class="token number">5</span><span class="token punctuation">,</span>
  name<span class="token operator">:</span> <span class="token string">'San Francisco'</span><span class="token punctuation">,</span>
  <span class="token punctuation">[</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token string">'enabled'</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码中，对象<code>obj</code>的最后一个属性名，需要计算得到。这时最好采用属性表达式，在新建<code>obj</code>的时候，将该属性与其他属性定义在一起。这样一来，所有属性就在一个地方定义了。</p> <p>另外，对象的属性和方法，<strong>尽量采用简洁表达法</strong>，这样易于描述和书写。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> ref <span class="token operator">=</span> <span class="token string">'some value'</span><span class="token punctuation">;</span>

<span class="token comment">// bad</span>
<span class="token keyword">const</span> atom <span class="token operator">=</span> <span class="token punctuation">{</span>
  ref<span class="token operator">:</span> ref<span class="token punctuation">,</span>

  value<span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>

  <span class="token function-variable function">addValue</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> atom<span class="token punctuation">.</span>value <span class="token operator">+</span> value<span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// good</span>
<span class="token keyword">const</span> atom <span class="token operator">=</span> <span class="token punctuation">{</span>
  ref<span class="token punctuation">,</span>

  value<span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>

  <span class="token function">addValue</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> atom<span class="token punctuation">.</span>value <span class="token operator">+</span> value<span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br></div></div><h2 id="数组"><a href="#数组" class="header-anchor">#</a> 数组</h2> <p>使用<strong>扩展运算符（...）拷贝数组</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">const</span> len <span class="token operator">=</span> items<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
<span class="token keyword">const</span> itemsCopy <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> i<span class="token punctuation">;</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span>i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> len<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  itemsCopy<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">=</span> items<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// good</span>
<span class="token keyword">const</span> itemsCopy <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span>items<span class="token punctuation">]</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>使用 <strong>Array.from 方法，将类似数组的对象转为数组</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> foo <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelectorAll</span><span class="token punctuation">(</span><span class="token string">'.foo'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> nodes <span class="token operator">=</span> Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span>foo<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="函数"><a href="#函数" class="header-anchor">#</a> 函数</h2> <p>立即执行函数可以写成箭头函数的形式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Welcome to the Internet.'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>那些使用匿名函数当作参数的场合，<strong>尽量用箭头函数代替</strong>。因为这样更简洁，而且绑定了 this。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> x <span class="token operator">*</span> x<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// good</span>
<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> x <span class="token operator">*</span> x<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// best</span>
<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">x</span> <span class="token operator">=&gt;</span> x <span class="token operator">*</span> x<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>箭头函数取代<code>Function.prototype.bind</code>，不应再用 self/_this/that 绑定 this。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">const</span> self <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> <span class="token function-variable function">boundMethod</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>params</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token function">method</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>self<span class="token punctuation">,</span> params<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// acceptable</span>
<span class="token keyword">const</span> boundMethod <span class="token operator">=</span> <span class="token function">method</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// best</span>
<span class="token keyword">const</span> <span class="token function-variable function">boundMethod</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>params</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token function">method</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> params<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>简单的、单行的、不会复用的函数，建议采用箭头函数。<strong>如果函数体较为复杂，行数较多，还是应该采用传统的函数写法。</strong></p> <p><strong>所有配置项都应该集中在一个对象，放在最后一个参数，布尔值不可以直接作为参数</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">function</span> <span class="token function">divide</span><span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span> b<span class="token punctuation">,</span> option <span class="token operator">=</span> <span class="token boolean">false</span></span> <span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token punctuation">}</span>

<span class="token comment">// good</span>
<span class="token keyword">function</span> <span class="token function">divide</span><span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span> b<span class="token punctuation">,</span> <span class="token punctuation">{</span> option <span class="token operator">=</span> <span class="token boolean">false</span> <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>不要在函数体内使用 arguments 变量，使用 rest 运算符（...）代替。因为 rest 运算符显式表明你想要获取参数，而且 arguments 是一个类似数组的对象，而 rest 运算符可以提供一个真正的数组。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">function</span> <span class="token function">concatenateAll</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> args <span class="token operator">=</span> <span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>arguments<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> args<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">''</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// good</span>
<span class="token keyword">function</span> <span class="token function">concatenateAll</span><span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> args<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">''</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>使用默认值语法设置函数参数的默认值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">function</span> <span class="token function">handleThings</span><span class="token punctuation">(</span><span class="token parameter">opts</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  opts <span class="token operator">=</span> opts <span class="token operator">||</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// good</span>
<span class="token keyword">function</span> <span class="token function">handleThings</span><span class="token punctuation">(</span><span class="token parameter">opts <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><h2 id="map-结构"><a href="#map-结构" class="header-anchor">#</a> Map 结构</h2> <p>注意区分 Object 和 Map，只有模拟现实世界的实体对象时，才使用 Object。如果只是需要<code>key: value</code>的数据结构，使用 Map 结构。因为 Map 有内建的遍历机制。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> map <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Map</span><span class="token punctuation">(</span>arr<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> key <span class="token keyword">of</span> map<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> value <span class="token keyword">of</span> map<span class="token punctuation">.</span><span class="token function">values</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> item <span class="token keyword">of</span> map<span class="token punctuation">.</span><span class="token function">entries</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>item<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">,</span> item<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><h2 id="class"><a href="#class" class="header-anchor">#</a> Class</h2> <p>总是用 Class，取代需要 prototype 的操作。因为 Class 的写法更简洁，更易于理解。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">function</span> <span class="token function">Queue</span><span class="token punctuation">(</span><span class="token parameter">contents <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">this</span><span class="token punctuation">.</span>_queue <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span>contents<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token class-name">Queue</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">pop</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> value <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>_queue<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token keyword">this</span><span class="token punctuation">.</span>_queue<span class="token punctuation">.</span><span class="token function">splice</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> value<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// good</span>
<span class="token keyword">class</span> <span class="token class-name">Queue</span> <span class="token punctuation">{</span>
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">contents <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>_queue <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token operator">...</span>contents<span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token function">pop</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> value <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>_queue<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>_queue<span class="token punctuation">.</span><span class="token function">splice</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> value<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><p>使用<code>extends</code>实现继承，因为这样更简单，不会有破坏<code>instanceof</code>运算的危险。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">const</span> inherits <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'inherits'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">function</span> <span class="token function">PeekableQueue</span><span class="token punctuation">(</span><span class="token parameter">contents</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token function">Queue</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> contents<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token function">inherits</span><span class="token punctuation">(</span>PeekableQueue<span class="token punctuation">,</span> Queue<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">PeekableQueue</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">peek</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>_queue<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// good</span>
<span class="token keyword">class</span> <span class="token class-name">PeekableQueue</span> <span class="token keyword">extends</span> <span class="token class-name">Queue</span> <span class="token punctuation">{</span>
  <span class="token function">peek</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>_queue<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><h2 id="模块"><a href="#模块" class="header-anchor">#</a> 模块</h2> <p>首先，Module 语法是 JavaScript 模块的标准写法，坚持使用这种写法。使用<code>import</code>取代<code>require</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">const</span> moduleA <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'moduleA'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> func1 <span class="token operator">=</span> moduleA<span class="token punctuation">.</span>func1<span class="token punctuation">;</span>
<span class="token keyword">const</span> func2 <span class="token operator">=</span> moduleA<span class="token punctuation">.</span>func2<span class="token punctuation">;</span>

<span class="token comment">// good</span>
<span class="token keyword">import</span> <span class="token punctuation">{</span> func1<span class="token punctuation">,</span> func2 <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'moduleA'</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>使用<code>export</code>取代<code>module.exports</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// commonJS的写法</span>
<span class="token keyword">var</span> React <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'react'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">var</span> Breadcrumbs <span class="token operator">=</span> React<span class="token punctuation">.</span><span class="token function">createClass</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
  <span class="token function">render</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token operator">&lt;</span>nav <span class="token operator">/</span><span class="token operator">&gt;</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

module<span class="token punctuation">.</span>exports <span class="token operator">=</span> Breadcrumbs<span class="token punctuation">;</span>

<span class="token comment">// ES6的写法</span>
<span class="token keyword">import</span> React <span class="token keyword">from</span> <span class="token string">'react'</span><span class="token punctuation">;</span>

<span class="token keyword">class</span> <span class="token class-name">Breadcrumbs</span> <span class="token keyword">extends</span> <span class="token class-name">React<span class="token punctuation">.</span>Component</span> <span class="token punctuation">{</span>
  <span class="token function">render</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token operator">&lt;</span>nav <span class="token operator">/</span><span class="token operator">&gt;</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">export</span> <span class="token keyword">default</span> Breadcrumbs<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><p>如果模块只有一个输出值，就使用<code>export default</code>，如果模块有多个输出值，就不使用<code>export default</code>，<code>export default</code>与普通的<code>export</code>不要同时使用。</p> <p>不要在模块输入中使用通配符。因为这样可以确保你的模块之中，有一个默认输出（export default）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// bad</span>
<span class="token keyword">import</span> <span class="token operator">*</span> <span class="token keyword">as</span> myObject <span class="token keyword">from</span> <span class="token string">'./importModule'</span><span class="token punctuation">;</span>

<span class="token comment">// good</span>
<span class="token keyword">import</span> myObject <span class="token keyword">from</span> <span class="token string">'./importModule'</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>如果模块默认输出一个函数，函数名的首字母应该小写。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">makeStyleGuide</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token punctuation">}</span>

<span class="token keyword">export</span> <span class="token keyword">default</span> makeStyleGuide<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>如果模块默认输出一个对象，对象名的首字母应该大写。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> StyleGuide <span class="token operator">=</span> <span class="token punctuation">{</span>
  es6<span class="token operator">:</span> <span class="token punctuation">{</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">export</span> <span class="token keyword">default</span> StyleGuide<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><h2 id="eslint-的使用"><a href="#eslint-的使用" class="header-anchor">#</a> ESLint 的使用</h2> <p>ESLint 是一个语法规则和代码风格的检查工具，可以用来保证写出语法正确、风格统一的代码。</p> <p>首先，安装 ESLint。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">npm</span> i -g eslint
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>然后，安装 Airbnb 语法规则，以及 import、a11y、react 插件。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ <span class="token function">npm</span> i -g eslint-config-airbnb
$ <span class="token function">npm</span> i -g eslint-plugin-import eslint-plugin-jsx-a11y eslint-plugin-react
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>最后，在项目的根目录下新建一个<code>.eslintrc</code>文件，配置 ESLint。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">{</span>
  <span class="token string">&quot;extends&quot;</span><span class="token operator">:</span> <span class="token string">&quot;eslint-config-airbnb&quot;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>现在就可以检查，当前项目的代码是否符合预设的规则。</p> <p><code>index.js</code>文件的代码如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> unusued <span class="token operator">=</span> <span class="token string">'I have no purpose!'</span><span class="token punctuation">;</span>

<span class="token keyword">function</span> <span class="token function">greet</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">var</span> message <span class="token operator">=</span> <span class="token string">'Hello, World!'</span><span class="token punctuation">;</span>
    <span class="token function">alert</span><span class="token punctuation">(</span>message<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token function">greet</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>使用 ESLint 检查这个文件，就会报出错误。</p> <div class="language-bash line-numbers-mode"><pre class="language-bash"><code>$ eslint index.js
index.js
  <span class="token number">1</span>:1  error  Unexpected var, use <span class="token builtin class-name">let</span> or const instead          no-var
  <span class="token number">1</span>:5  error  unusued is defined but never used                 no-unused-vars
  <span class="token number">4</span>:5  error  Expected indentation of <span class="token number">2</span> characters but found <span class="token number">4</span>  indent
  <span class="token number">4</span>:5  error  Unexpected var, use <span class="token builtin class-name">let</span> or const instead          no-var
  <span class="token number">5</span>:5  error  Expected indentation of <span class="token number">2</span> characters but found <span class="token number">4</span>  indent

✖ <span class="token number">5</span> problems <span class="token punctuation">(</span><span class="token number">5</span> errors, <span class="token number">0</span> warnings<span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码说明，原文件有五个错误，其中两个是不应该使用<code>var</code>命令，而要使用<code>let</code>或<code>const</code>；一个是定义了变量，却没有使用；另外两个是行首缩进为 4 个空格，而不是规定的 2 个空格。</p></div></div> <div class="page-slot page-slot-bottom"><!-- 横向自适应 -->
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